The vertebrae of the vertebral column each have a spinous process, with the exception of the fused sacral and coccygeal vertebrae, where this spinous process is a more or less discernible vestige. These spinous processes have materially the shape of a plate oriented in the sagittal plane, with an upper edge, a lower edge, two lateral faces and a crest pointing rearward of the patient. The edges and faces can be more or less rounded depending on the patient, but make it possible to define surfaces on which an implant can bear. The interspinous space that separates two adjacent spinous processes has a variable size depending on the patient and on the location within the vertebral column. This space is generally filled by an interspinous ligament. In addition, the crests are covered by the supraspinous ligament that runs over the entire vertebral column. The spinous processes are therefore attached to one another by interspinous ligaments and the supraspinous ligament.
Interspinous implants are generally used particularly for pathologies such as herniated discs or degenerative lesions, hyperlordosis (particularly in the aged), shrinkage of the vertebral canal (lumbar stenosis) treated without laminectomy, persistent chronic lumbar pain, particularly in the event of failure of conservative treatment, degenerative disease of an intervertebral disc in a location adjacent to a prior fusion, spondylolisthesis of a grade lower than 1, etc. In certain cases, particularly the least severe pathologies, the interspinous implant provides an alternative or preliminary measure to more invasive measures. Thus, the implant may be used alone to maintain or restore the interspinous space to physiological values, particularly by preserving the mobility of the spinous processes in certain cases. In other cases, the interspinous implant can be used for vertebral fusion, particularly of the spinous processes but also in association with a disc fusion, for example.
Interspinous implants should make it possible to restore elevation between spinous processes and to maintain that elevation (to relieve articular facets, the nerve roots, the disc, etc.), particularly while awaiting arthrodesis (bone fusion). They must be stable between spinous processes, particularly in the lumbar region, whether or not having means available of anchoring to the spinous processes. In addition, it is desirable to have different implant heights and/or widths and/or depths available, particularly to match them best to their implantation sites. In addition it is preferred, in some cases, to immobilize the two spinous processes, while in other cases it is preferable to retain mobility. It is also sometimes desirable to control the extent of mobility, particularly bending and rotation motion of the vertebrae.
One problem relates to the stability of interspinous implants once implanted between two spinous processes. The implant must not become dislodged from its setting between the two spinous processes.
The stability of the interspinous implant is most often conferred by lateral wings, blades, arms or legs whose width makes it possible for them to spread over a relatively large area of the two adjacent spinous processes. The interspinous implants should, however, be able to lend themselves to multi-level surgery, and it is desirable that two implants be implantable in two adjacent interspinous spaces. The implant must therefore be stable between spinous processes, yet without having too great a bulk, so as to facilitate its implantation and/or allow its use in adjacent interspinous spaces.
Another problem relates to the invasiveness of the implantation. As it happens, it is generally preferable for the implant to be easily implantable and that it not be necessary to free too great a space in the interspinous space to be treated. It is also generally preferable to leave intact as many of the structures around the implantation site as possible. A compact implant, therefore, would be desirable.
Implants including at least one body insertable through the interspinous ligament are known from the prior art. Certain known implants necessitate, for their implantation, the total removal of the interspinous ligament located between the two spinous processes involved in the implantation, but sometimes also of at least a portion of the interspinous ligaments of the adjacent spinous processes. Certain implants also necessitate the removal of the supraspinous ligament, at least in the portion located over the two spinous processes involved in the implantation. These ligament removals are not desirable for the patient because they risk destabilizing the vertebral column and hence the implant, for example by increasing the risks of excessive motion of the spinous processes. Finally, these implants require the opening of too large an approach path for their insertion, usually by pushing away the surrounding tissues, which is not desirable for the patient for the same reasons as well as for reasons of muscle dilapidation.
Therefore, to obtain the least invasive implant possible, not requiring the opening of a large approach path, it is necessary to reduce the bulk of the implant and to limit the size of the implantation path. These constraints are attended by a problem of difficulty in implantation, especially if one wants to open only one (side) face of access to the spinous processes. In fact, for the least invasive possible implantation for example, an approach path passing only through a plane near the sagittal plane of the spinous processes might be desired, allowing impingement for instance only on one side of the patient's vertebral column, that is to say on only one lateral face of the spinous processes.
It will be noted that addressing the problem of invasiveness contributes additional constraints to the problem of stability, in particular because reducing the dimensions for reducing invasiveness may induce risks of stability. In this context, it is advantageous to offer a solution which allows invasiveness and stability to be reconciled.